Method for manufacturing a bipolar plate

ABSTRACT

Described is a method for manufacturing a bipolar plate for, for instance, a bipolar battery. According to the present invention, a negative plate is manufactured by applying a suitable paste in a metal grid, and a positive plate is manufactured by applying a suitable paste in a metal grid. The pasted plates are formed, and arranged on opposite sides of a suitable platelike substrate. The substrate is thermally pressed, then a surface activating treatment is carried out, and thereafter the substrate is lead-plated. That surface activating treatment can be carried out with a corona or, preferably, a plasma. To ensure a good electrical contact, the substrate can be roughened, and the plates can be pressed against the substrate by utilizing a compressed glass fiber mat.

BACKGROUND OF THE INVENTION

Dutch patent application 10.03340 of older date and of the same inventoras the present application describes a bipolar battery which hasimproved performance over existing batteries. More particularly, thatearlier patent application describes a bipolar plate basicallyconsisting of a suitable substrate provided on either side with activelayers, the substrate basically consisting of a graphite felt filledwith a plastic. The contents of that earlier application are consideredto be incorporated herein by reference.

The earlier application describes two ways of manufacturing a bipolarplate based on the composite substrate described in that earlierapplication. A first procedure involves the application of a negativepole paste, known per se, to a main surface of the substrate for forminga negative pole layer, and the application of a positive pole paste,known per se, to the opposite main surface of the substrate for forminga positive pole layer. A second procedure, specifically for the purposeof a bipolar plate for use in a lead-acid battery, involves firstmanufacturing a lead-plated composite substrate, and subsequentlyactivating the lead-plated composite substrate by carrying out theso-called Plante process in a special manner, whereby the substantiallymassive lead of one surface is converted to a porous lead grid, whilethe substantially massive lead of the other surface is converted to aporous grid of PbO₂.

It has been experimentally established that the first-mentioned processis able to provide batteries having a good performance. It is a problem,however, to implement this process on a commercial scale, because forthat purpose special machines must be developed, a practical problembeing that the two opposite surfaces of the substrate must be processedin different ways. More particularly, the two pastes must be formed indifferent ways. Moreover, it is a drawback that in the application ofpaste to a flat plate, adhesion is not optimal and the capacity is notoptimally utilized; to remove this drawback, the substrate would have tobe provided with a grid on both sides.

The second process has, as such, good utility for manufacturing apractically applicable bipolar plate. It has been experimentallyestablished, however, that the performance of the thus manufacturedbatteries, at high rates of discharge (high discharge currentintensities), lags behind the performance of batteries manufacturedaccording to the first-mentioned process. Further, it is a problem thatin the Plante process described, a strong corrosion occurs of the thinlead films for forming the porous lead, which corrosion isdisadvantageous to the substrate.

SUMMARY OF THE INVENTION

The object of the present invention is therefore to provide a method formanufacturing a bipolar plate for, for instance, a bipolar battery,which method can be implemented in a relatively simple and inexpensivemanner, while batteries constructed with the thus manufactured bipolarplates have a performance which, in particular at high rates ofdischarge, is comparable to, or even better than, the performance ofknown starter batteries.

In starter batteries presently known, use is made not of bipolar platesbut of monopolar plates, which are formed as thin, pasted plates. Inmanufacturing monopolar plates, use can be made of a separatemanufacturing machine for positive plates, as well as of a separatemanufacturing machine for negative plates, that is, the positive platesand the negative plates are manufactured independently of each other.This provides the advantage that the manufacturing processes used formanufacturing positive plates and for manufacturing negative plates canbe optimized independently of each other.

It is customary to manufacture monopolar plates in the form of a metalgrid in which a paste is applied, which paste is subsequently formed.During the forming operation, the metal grid serves as a matrix to keepthe paste in position. During use, the metal grid serves on the one handto give the plate mechanical integrity and on the other for currentconduction. Because in monopolar plates the current is drawn via a sideedge of the plate, which means that the direction of the electriccurrent within the plate is parallel to the plate surface, the metalgrid should be of rather heavy design. In a practical example, the gridis constructed in the form of horizontal and vertical bars whosethickness is in the order of a few millimeters, which bars encloserectangular spaces of about 5×10 mm², in which spaces the paste isapplied. The mass ratio of the paste to the grid is approximately 1:1.

More specifically, therefore, the object of the invention is to providea method for manufacturing a bipolar plate, as well as a bipolar batteryand a method for manufacturing same, in which the advantages of existingtechniques are combined as much as possible, while the disadvantages ofexisting techniques are eliminated as much as possible.

According to an important aspect of the present invention, in a methodfor manufacturing a bipolar plate, use is made of separatelymanufactured pasted plates, which are applied to the substrate orpressed onto it. This has the advantage, in the first place, that usecan be made of existing techniques for manufacturing pasted plates.However, because in a bipolar plate the current is drawn in a directionperpendicular to the surface, the current conducting properties of thegrid of the pasted plates (for current conduction in a directionparallel to the surface) need to meet less stringent requirements.Moreover, the pasted plates are supported over their entire surface bythe substrate, so that they do not need to be self-supporting anymore.The above-mentioned two aspects imply that the metal grid of the pastedplates can be considerably less heavy than in the case of monopolarplates, which means that a greater percentage of the weight of thepasted plates can consist of paste, so that the performance per unit ofweight improves. A paste-to-grid weight ratio of 3:1 seems realizablewithout any problems, but even a ratio of 10:1 is envisaged.

A further aspect of the present invention is directed to the provisionof a substrate, as well as to the provision of a method formanufacturing a substrate, which substrate is better suited than knownsubstrates to be used in combination with pasted plates placed againstit. According to this further aspect of the present invention, thesubstrate is provided with a lead film, in order to ensure the bestpossible contact between the substrate on the one hand and the pastedplates on the other. For a good quality of the lead films, it ispreferred that the substrate is first thermally pressed, and thethermally pressed substrate is subsequently subjected to a surfaceactivating treatment, whereafter the lead film is applied.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned aspects and other aspects, features and advantagesof the present invention will be clarified by the following descriptionof preferred embodiments of a method according to the invention, withreference to the drawings, in which equal reference numerals denoteequal or comparable parts, and wherein:

FIGS. 1A-B schematically illustrate different stages of a method forassembling a bipolar battery;

FIG. 2 schematically illustrates a 12V battery manufactured according tothe present invention;

FIGS. 3A-3C7 illustrates construction details of a battery;

FIG. 4 illustrates a detail of a battery which, according the presentinvention, is provided with internal cooling;

FIG. 5 schematically illustrates an arrangement for carrying out asurface activating treatment using a corona;

FIG. 6 schematically illustrates a peeling test; and

FIG. 7 schematically illustrates an arrangement for carrying out asurface activating treatment using a plasma.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1A schematically shows a cross section of the most important threeparts of a bipolar plate 10, viz. a platelike substrate 11, a negativeplate 112 and a positive plate 113. For clarity, these three parts aredrawn as loose parts in FIG. 1A.

The platelike substrate 11 contains a matrix 15 of carbon fibers 14without preferred direction, which is designated as a non-woven carbonmat or carbon felt. The carbon fibers 14 have a thickness in the orderof approximately 8 μm, and are preferably graphitized, i.e. at leastpartly, but preferably wholly, converted to graphite. By virtue of thecombination of these features, the matrix 15 has a particularly goodconductivity in all three spatial directions, in particular in thedirection perpendicular to the surface of the platelike substrate 11.

The matrix 15 itself is particularly porous: the porosity is about 95%.The use of this material for a bipolar plate 10 is therefore notobvious. The substrate 11 is to a proper extent liquid-tight, however,in that a plastic material 16 has been applied in the pore spaces of thematrix 15. Preferably, the plastic material 16 is a conductive plastic,so that the electrical conductivity is further improved.

The combination of the matrix 15 of carbon fibers 14 and the plasticmaterial 16 filling the pores of the matrix 15 is also designated ascomposite substrate. This combination possesses a particularly goodmechanical stability.

The composite substrate 11 is a particularly suitable basic material formanufacturing a bipolar plate 10 and can be manufactured as follows. Ina first step, a carbon felt is provided. This can be effected, forinstance, by making a suspension of carbon fibers in a medium such aswater, and subsequently pouring this suspension onto a rotating drum.After evaporation of the medium, the porous carbon felt remains behindon the drum, and that felt possesses intrinsically sufficient strengthto be taken off the drum.

In a second step, a thermoplastic plastic, preferably a conductivepolymer, is applied in the pores of the carbon felt. This can beeffected, for instance, by preparing a solution of a thermoplasticplastic such as fluoelastomer in an organic solvent such astetrahydrofuran or acetone. For obtaining the desired conductivity ofthe plastic, it is preferred to add synthetic graphite powder to thatsolution, whose particles can possess typical dimensions in the range of1-2 μm. This suspension is applied to the carbon felt, whereby it isabsorbed into the pores of the carbon felt. The solvent can be removedby drying in air (evaporation), but preferably the solvent is removed bydrying under reduced pressure (a few millibars) and at elevatedtemperature (about 80° C.).

In connection with the composite substrate 11, reference can be made,for instance, to the publications WO 96/12313 and JP-61.177.386.

The negative plate 112 comprises a grid 120 and a negative paste 121applied in the grid 120, which paste 121 is formed. As alreadymentioned, a known grid is constituted by horizontal and vertical posts.The negative plate 112 can be manufactured identically to monopolarnegative pasted plates known per se, with the understanding that thegrid 120 can be less heavy than is conventional in monopolar pastedplates. The positive plate 113 comprises a grid 122 and a positive paste123 applied in the grid 122, which paste 123 is formed. The positiveplate 113 can be manufactured identically to monopolar positive pastedplates known per se, with the understanding that the grid 122 can beless heavy than is conventional in monopolar pasted plates. Thus, forthe provision of the plates 112 and 113, use can advantageously be madeof existing techniques, with the understanding that a grid can be usedwhose posts can be thinner than in presently known grids, so that theweight ratio of paste to grid can be better than 3:1, and can even be10:1.

In FIG. 1A, for the sake of clarity, the substrate 11 and the plates 112and 113 are represented as loose parts. According to an important aspectof the present invention, it is possible to unite the above three partsto form a bipolar plate 10 in a relatively simple manner, viz. byplacing the plates 112 and 113, respectively, against the respectivemain surfaces 17 and 18 of the substrate 11.

For obtaining a good contact between the substrate 11 and the plates112, 113, the main surfaces 17 and 18 are preferably provided with ametal film 31 and 32, respectively, preferably a lead film, as will bedescribed in more detail later.

Further, it is preferred to ensure that the surfaces 17 and 18 (whichmay or may not be lead-plated) of the substrate 11 have a certain degreeof roughness before the plates 112 and 113 are placed against thosesurfaces 17 and 18. If the surfaces 17 and 18 are not intrinsicallyrough to a sufficient extent, for instance because lead-plating hasyielded smooth surfaces, those surfaces can be roughened in a separatestep. Preferably, the surfaces 17 and 18 have a roughness of at leastabout 10-20 μm.

Ensuring the desired roughness can be effected in different ways. Afterthe above-mentioned second step, wherein the thermoplastic plastic isapplied in the pores of the carbon felt, the substrate has a certaindegree of intrinsic roughness because of the graphite fiber endsprojecting from the surface. However, as will be explained in moredetail hereinbelow, after the second step (and after drying) thesubstrate is preferably thermally pressed before the lead film isapplied. As a result of thermal pressing, the surface of the substrateis virtually completely flat, as is a metal film applied to it. Forproviding the desired roughness, that metal film can then be worked, forinstance mechanically (for instance by irradiation with pellets ofsuitable dimensions and hardness: “shot-peening”), electrochemically(for instance by a chemical deposition process in a plating bath) orphysically (for instance by spraying with a lead mist for obtaining aporous film, as is known per se, for instance, from U.S. Pat. No.5,527,642).

Further, it is preferred to ensure that the plates 112 and 113 bepressed against those surfaces 17 and 18 with a certain force. FIG. 1Bshows a schematic cross section of a battery 1, in which the desiredextent of pressing-on is automatically provided. The battery 1 comprisesa container 2 with sidewalls 6, in which the above-described bipolarplate 10 according to FIG. 1A is placed. Arranged opposite the negativeplate 112 is a positive monopolar plate 4, which can be identical to thepositive plate 113. Arranged opposite the positive plate 113 is anegative monopolar plate 5, which can be identical to the negative plate112. Arranged between the negative plate 112 and the positive monopolarplate 4 is a glass fiber separator 21, and arranged between the positiveplate 113 and the negative monopolar plate 5 is a glass fiber separator22, which can be identical to the glass fiber separator 21, whichseparators 21 and 22 are filled with an electrolyte. The monopolarplates 4 and 5 are (directly or indirectly) supported by the walls 6 ofthe container 2. The thickness dimensions of the separators 21 and 22are chosen such that upon placement of the above-mentioned parts in thecontainer 2, those separators 21 and 22 are elastically compressed to acertain extent. Preferably, the extent of compression is about 30 to35%. Through that elastic compression, the separators 21 and 22 exert toa sufficient extent a press-on force on the plates 112 and 113.

It is also possible to place several bipolar cells in series, as will bedescribed later.

The above-described example involves a physical contact between thesubstrate 11 and the plates 112 and 113. It is also possible, however,to effect a chemical connection between the substrate 11 and the plates112 and 113. In a preferred embodiment of the method according to thepresent invention, this is effected in a relatively simple manner byplacing the plates 112 and 113 under a pressure against the substrate11, directly after the paste 121 and 123 is provided in the grid 120 and122, that is, before the curing process takes place. The chemicalconnection between the substrate 11 and the plates 112 and 113 is thencreated automatically as a result of that curing process.

EXAMPLE

Below follows a description, with reference to FIGS. 2 and 3, of theresults of a test arrangement in which the proposals of the presentinvention were tested utilizing standard available components accordingto the state of the art, that is, not yet optimized components.

In this test, a 12V battery 100 was constructed with the configurationillustrated in FIG. 2. Successively arranged in series in a container 2were a positive monopolar plate 4, five bipolar plates 10 ₁ through 10₅, and a negative monopolar plate 5. The plates 4, 10 ₁ through 10 ₅,and 5 were separated from each other by six glass fiber separators 21 ₁through 21 ₆. For the negative monopolar plate 5 and the positivemonopolar plate 4, use was made of two, priorly formed, conventionalstarter plates of a thickness of 1.7 mm. The bipolar plates 10 ₁ through10 ₅ were mutually identical, and had the structure described withreference to FIGS. 1A-B. In each bipolar plate, use was made of alead-plated composite substrate 11 as described in the earlier Dutchpatent application 10.03340. For the negative plates 112, use was madeof priorly formed starter plates that were identical to the negativemonopolar plate 5. For the positive plates 113, use was made of priorlyformed starter plates that were identical to the positive monopolarplate 4. The six glass fiber separators 211 through 216, prior tocompression, each had a thickness of 2×1.17 mm, and they were compressedby about 32% upon placement in the container 2 for assembling theconfiguration illustrated in FIG. 2.

The effective surface of the plates 4, 10 ₁ through 10 ₅, and 5 was 47cm²; the internal resistance of the battery 100, in fully chargedcondition of the battery, was 0.021 Ohm.

It is noted that for the purpose of drawing current over the entiresurface of the monopolar plates 4 and 5, copper end plates had beenplaced between those monopolar plates 4 and 5 and the correspondingwalls 6 of the container 2, but for the sake of simplicity this is notshown in FIG. 2.

The capacity of the thus constructed test battery 100 was measured as afunction of the discharge current density. At a specific energy of 5Wh/kg, the specific power was found to be 500 W/kg. The last-mentionedfigures correspond to a discharge time of 36 sec, that is, a C-rate of100 W/Wh (ratio of specific power/specific energy).

As mentioned, in the described test arrangement, use was made ofstandard available components, in particular standard available starterplates 4, 5, 112, 113; with these, already a specific power of 500 W/kgwas achieved in a simple manner. These standard available starterplates, however, are optimized for their current application, viz.monopolar use, whereby the plates are self-supporting and the current isdrawn along an upper edge, for which reason the grids of the plates mustbe of rather thick and heavy design. This means that optimization forthe application proposed by the present invention is possible, and whatis specifically envisaged is making the grid of the starter plateslighter. As a result, the performance will improve by two effects. Onthe one hand, the specific power and the specific energy will increasein that the ratio of paste weight to grid weight increases. On theother, a plate can contain more active paste in that the grid occupiesless space.

In a particular embodiment, the present invention proposes manufacturinga pasted plate based on a metal foam, more particularly a lead foam or alead alloy foam, with the active paste applied in the pore spaces of thefoam; in other words: the grid is formed by metal foam. Metal foams areknown per se, and have a porosity normally in the order of 96%. A pastedplate manufactured according to this proposal has a particularlyfavorable ratio of paste weight to total weight.

Another possibility for optimization is the improvement of theelectrical contact between the substrate 11 and the starter plates 112and 113 pressed against it.

By and large, it is expected that the test results mentioned can beimproved by at least a factor of 2 through optimization of thecomponents used. From the foregoing, it appears that a practicallyapplicable battery according to the present invention will be attainablebecause for acceleration purposes for electrical (hybrid) vehicles aminimal specific power of 500 W/kg is a requirement, which is alreadyattainable with the current standard components.

FIG. 3 shows more details of the construction of the tested battery.FIG. 3A shows a schematic perspective view of an exemplary embodiment ofa spacer 200; FIG. 3B shows a cross section taken on the line B—B inFIG. 3A; and FIG. 3C shows a longitudinal section of a portion of thebattery 100 of FIG. 2.

The spacer 200 is made of PE, but a different suitable plastic will alsobe satisfactory. The spacer 200 is generally in the form of arectangular block, of a thickness D of about 5 mm, a height of about 13cm and a width of about 9 cm. In the example shown, the spacer 200 has afirst main surface (front face) 201, a second main surface (back face)202, and a top surface 203. Starting from the first main surface 201,there are provided in the body of the spacer 200, two substantiallyrectangular recesses 211 and 212, which communicate via a communicationrecess or passage 213. The two recesses 211 and 212 can be of equaldepth, as shown, but that is not requisite.

In the bottom of the second recess 212 a substantially rectangular hole214 is formed, whose dimensions are smaller than those of the secondrecess 212, and which is substantially centered relative to the secondrecess 212. The remaining portion of the bottom of the second recess 212constitutes a framelike supporting edge 215, extending around therectangular hole 214, of a thickness of about 1.5 mm. In the embodimenttested, the rectangular hole 214 had dimensions of 6×7 cm², and thedimensions of the second recess 212 were 7×8 cm², so that the supportingedge 215 had a width of 5 mm. Further, the dimensions of the firstrecess 211 were about 7×2 cm².

FIG. 3C illustrates the manufacture of a construction unit for abattery. First, on a suitable support (not shown) a first compositesubstrate 11 ₁ is placed, on which in turn the spacer 200 shown in FIGS.3A-B is placed (FIG. 3C.1). The spacer 200 then has its back face 202resting on a main surface 17 of the substrate 11 ₁.

Next, in the hole 214 a pasted plate is positioned, whose dimensionssubstantially correspond to those of the hole 214, and whose thicknessis approximately 1.7 mm (FIG. 3C.2). In the example tested, the pastedplate placed in the hole 214 was a negative pasted plate 112. It will beclear, however, that the pasted plate placed in the hole 214 can also bea positive pasted plate.

Next, in the second recess 212, a porous, elastic, insulating separator21 is positioned, whose dimensions substantially correspond to those ofthe second recess 212 (FIG. 3C.3). In the embodiment tested, for theseparator 21, use was made of two superposed sheets of glass fiber mat,each having a thickness of 1.17 mm.

Next, in the second recess 212, on the separator 21, a second pastedplate is positioned, whose dimensions substantially correspond to thoseof the second recess 212, and whose thickness is approximately 1.7 mm(FIG. 3C.4). Because the first pasted plate is a negative pasted plate,the second pasted plate in the example discussed here is a positivepasted plate 113.

Finally, on the thus formed stack a second composite substrate 11 ₂ isplaced (FIG. 3C.5). Because the sum of the thicknesses of the pastedplates 112, 113 and the separator 21 (5.74 mm) is greater than thethickness of the spacer 200 (5 mm), the second pasted plate 113 projectsbeyond the front face 201 of the spacer 200, and the second compositesubstrate 11 ₂ is clear of the spacer 200, as shown.

For manufacturing a battery with multiple cells, the above-mentionedprocess is repeated, whereby a spacer 200 is placed on the secondcomposite substrate 11 ₂, etc. (FIG. 3C.6).

When the desired number of cells has been achieved, on the uppermostcomposite substrate 11 an end plate is placed, which is suitable forfitting a current drawing connection; the end plate can be, forinstance, a copper plate. It is noted that such an end plate is alsodisposed under the lowermost composite substrate, although this is notshown in the figures.

The thus formed package is then compressed, whereby all free spacebetween the substrates 11 and the spacers 200 is eliminated (FIG. 3C.7).The thickness of the separators 21 is thereby reduced from 2.34 mm to1.6 mm, which amounts to a compression of (2.34-1.6)/2.34=31.6%. In thisstage the circumferential edges of the successive spacers 200 andsubstrates 11 are aligned relative to each other and pressed againsteach other, and can be joined together for obtaining a liquid-tightjoint (sealing). For this purpose, use can be made, for instance, ofultrasonic welding, thermowelding, or other methods which are known perse. The sealing process can be effected, in accordance with theinventive concept of the invention, in a fairly simple manner becausethe components to be joined together all contain plastic.

The package is now placed in a housing 2, which is at least suitable fortaking up a force perpendicular to the surface of the plates. The platesare vertically oriented, with their respective top surfaces 203 facingup. The battery can now be filled with a suitable electrolyte, such asaccumulator acid. To that end, in each top surface 203 a filling orifice204 is formed, which is in communication with the first recess 211. viathe filling orifice 204, the first recess 211 and the passage 213, theelectrolyte reaches an electrolyte space which is defined by the secondrecess 212 of the spacer 200 in question. The amount of electrolyteprovided in each electrolyte space 212 is such that the liquid leveljust reaches the passage 213.

During filling, the electrolyte is absorbed into the separator 21,which, as a result, will tend to swell up, so that the pasted plates112, 113 are subject to a press-on force presses those pasted platesagainst the adjacent substrates 11 ₁, 11 ₂. The housing 2 must be ableto take up these press-on forces, as well as the forces caused by volumeexpansion of the active mass at discharge.

It is noted that in the concept discussed, it is virtually impossiblefor a short-circuit to arise between the pasted plates 112, 113 within aspacer 200. Such a short-circuit, which might in principle occur at theedges of the pasted plates, is avoided in the concept discussed in thatthe separator 21 is greater than either of the pasted plates (112).

The battery is now basically finished. During use, the electrolyte canwarm up and expand, and/or gas bubbles can be formed. To be able toaccommodate these phenomena without problems, the first recess 211functions as an expansion space. To prevent electrolyte from splashingout through the filling orifice 204, the position of the filling orificeis preferably offset relative to the position of the passage 213, asshown. The filling orifice 204 can be closed by securing a plug therein,but it is also possible to place a pressure relief valve (not shown) inthe filling orifice 204. Under normal circumstances, that pressurerelief valve is closed, so that no electrolyte will evaporate and thebattery does not need to be replenished (free of maintenance).

FIG. 4 shows a detail of a battery 100′ which, according to the presentinvention, is provided with an internal cooling. In a comparable mannerto that discussed with reference to FIG. 3, this battery 100′ comprisesa stack of spacers 200, with substrates 11 between them, while in thespacers 200 a sandwich of pasted plates 112, 113 and a separator 21 isarranged. Arranged at one or more points in the battery 100′, betweentwo adjacent spacers 200, is a sandwich of two substrates 11 with anelectrically conductive cooling plate 300 between them. The coolingplate 300 can be manufactured, for instance, from aluminum or titanium.In the cooling plate 300, channels (not shown, for clarity) can beprovided, in which a cooling medium such as air or preferably water canflow, to dissipate the heat produced in the battery 100′.

Presently, reference is made to FIG. 1A again.

As has been discussed in the foregoing, a bipolar plate 10 according tothe present invention is formed by placing monopolar pasted plates 112and 113 against the main surfaces 17 and 18 of a platelike substrate 11.An important aspect here is that there must a proper contact between theplatelike substrate 11 and the pasted plates 112, 113, that is, a lowestpossible transition resistance, for which reason the main surfaces 17,18 of the platelike substrate 11 are preferably metallized, morepreferably lead-plated. The reference numeral 11 is used hereinafter forthe composite substrate proper, that is, the combination of the carbonfiber felt 14 with the polymer 16, while the lead-plated substrate, thatis, the combination of the substrate 11 proper and the lead films 31 and32, will hereinafter be designated by the reference numeral 30.

The lead films 31 and 32 provide for the proper contact between theplatelike substrate 11 and the pasted plates 112 and 113, respectively.A particular aspect of the present invention concerns a method forproviding a metal film on the substrate 11, so as to yield a goodmechanical and electrical contact between the substrate 11 and thatmetal film. This aspect of the present invention will hereinafter beexplained in more detail for the case of the lead films 31 and 32, butit will be clear that the present invention is not limited to films oflead.

The lead films 31, 32 to be provided on the substrate 11 must satisfyvarious requirements. For instance, it is desired that each lead film31, 32 exhibits proper adhesion to the substrate 11; that each lead film31, 32 is a homogeneous and closed film; and that each lead film 31, 32is relatively thin. In the prior art, no means are described forrealizing these wishes simultaneously. Dutch patent application 10.03340of older date, mentioned above, describes a method for manufacturing alead-plated substrate, in which through a galvanic fluorate bath a leadfilm of approximately 100 μm is applied to the substrate, whereafter thelead-plated substrate is compressed to allow the fluoelastomer todeliquesce so as to form a continuous whole. A problem here is that thefluopolymer of the substrate has a hydrophobic character, while a metal,in particular lead, is hydrophilic. During lead-plating, the leadtherefore deposits preferentially on the carbon fiber ends sticking outof the felt, which, as it were, function as nuclei of deposition, witheach nucleus growing into a lead island. During thermal pressing afterthe galvanic lead-plating, those lead islands are then slightlycompressed. Each island exhibits good adhesion to a carbon fiber, andeventually a good adhesion between the lead film and the felt isobtained, because the surfaces of the felt are relatively rough, with,specifically, many carbon fiber ends projecting from the felt duringgalvanic lead-plating. It proves to be difficult, however, to ensure insuch a process that the lead film is fully closed. If the lead film isnot fully closed, the underlying material, in particular the graphite,can be affected by corrosion, which limits the practical life of thebattery.

One possibility of nonetheless obtaining a closed lead film is to givethe lead film a greater thickness. In a test, a lead film of a thicknessof 600 μm was applied to the substrate. While this film proved to beclosed, such a great thickness of the lead film has as a disadvantagethat the specific electrical performance of a battery manufactured withsuch a lead-plated substrate (expressed in electrical performance perunit of weight) is relatively low.

The present invention provides a solution to these problems in that in amethod according to the present invention the composite substrate 11 isfirst thermally pressed, and only then lead-plated, with the pressedcomposite substrate 11 being subjected to a surface activating treatmentbefore the electroplating process is carried out.

In an embodiment which has proved suitable, the composite substrate 11,after the second step of applying and drying the fluoelastomer, wascompressed for approximately 10 min at a pressure of about 200 kg/cm² ata temperature of about 150° C. As a result, the fluoelastomerdeliquesces to form a closed whole, that is, a liquid-tight fill-up ofthe pores of the carbon felt. The pressing temperature was chosen suchas to be higher than the melting temperature of the fluoelastomer used,which in this experiment was about 110° C., but lower than thedegradation temperature of the fluoelastomer used, which in thisexperiment was about 400° C.

After the thermal pressing process, the outer surface of the substrate11 consists substantially completely of fluopolymer 16, which gives thatouter surface a hydrophobic character, so that lead would in principleadhere poorly to such an outer surface of the substrate 11. Accordingly,it is not obvious to carry out the thermal pressing prior tolead-plating.

The surface activating treatment mentioned serves to change thehydrophobic character of the outer surface to a hydrophilic character toimprove the adhesion of the lead. To that end, the surface activatingtreatment should be able to modify in a suitable manner thefunctionality of certain groups of the plastic material, such as CFgroups, which determine the hydrophobic character. In the following, twoexamples of such a surface activating treatment will be discussed.

A first example of such a surface activating treatment utilizes a coronadischarge. An arrangement 400 for carrying out a surface activatingtreatment using a corona is shown schematically in FIG. 5. On a platform401 rests a (pressed) substrate 11 to be treated. The platform 401 ismovable, as indicated by the arrow P. Arranged above the substrate 11 isa high-voltage electrode 402, which in a test arrangement was in theform of a round cylinder of aluminum oxide, of a length of about 15 cmand a diameter of about 2 cm, having a metal conductor therein. To thehigh-voltage electrode 402 a high voltage HV is applied, as a result ofwhich a continuous gas discharge 403 is generated, with the platform 401functioning as counterelectrode. In this gas discharge (corona) 403,particularly reactive oxygen radicals are formed, which “attack” thesurface of the substrate 11. In a test, an alternating current of 22 kVwas applied to the high-voltage electrode 402, and a discharge of 10W/cm² was generated. The gas atmosphere was air, and was blown over thesurface of the substrate 11 (compressed air) to supply fresh oxygen andto accomplish a cooling effect. The distance between the sprayer 402 andthe substrate 11 to be treated was approximately 1-2 mm. The substrate11 had been manufactured and pressed in the manner describedhereinbefore; the polymer contained 50% graphite powder. The platform401 was advanced at a speed of 0.5 mm/sec. The thus treated substrate11, after completion of the corona treatment, was directly subjected toan electroplating operation in a standard fluoborate bath to deposit alead film 31. Alternatively, for instance a standard lead sulfonic acidbath can be used. The temperature of the plating bath was held at about22° C. The plating bath was operated in a pulsated fashion: for 30 sec acurrent of 12.5 mA/cm² was applied, whereafter for 0.1 sec a current ofequal direction but of a strength of 37.5 mA/cm² was applied. Thisprocess was continued for 35 minutes, so that the deposited lead film 31obtained a thickness of 25 μm.

Analysis of the formed lead film 31 demonstrated that this lead film 31exhibited a good homogeneity and was closed virtually completely (betterthan 99%). This result is considerably better than the result that isachieved if lead-plating occurs before the substrate is thermallypressed: then the lead film is certainly open for tens of percents.

The adhesion of the lead film 31 was tested by means of a peeling testto be discussed hereinafter with reference to FIG. 6. The film enduredthis test unaffected.

In the peeling test, the lead film 31, after being degreased, had astrip of self-adhesive tape 40 applied to it. An end 41 of this tape 40was pulled loose, the loose end 41 of the tape 40 making an angle ofabout 90° with the substrate surface, as outlined in FIG. 6. On theloose end 41 of the tape 40 a tensile force F was exerted, likewiseperpendicular to the substrate surface, and thus the tape 40 was pulledloose. The tape 40 used was of the type TESA 4541, and is manufacturedby the firm of Beiersdorf AG of Hamburg, Germany. Given a tape width of5 cm, a tensile force of 566 N was needed to pull the tape 40 off thelead film 31. A lead film 31 which had been applied to a surface treatedaccording to the present invention endured the peeling test unaffected,which is meant to say that no parts of the lead film 31 stuck to thetape. For comparison: when a substrate was lead-plated without theabove-described surface treatment, in a peeling test large portions ofthe lead film (more than 50%) were pulled off the substrate.

A second example of a suitable surface activating treatment utilizes aplasma. An arrangement 500 for carrying out a surface activatingtreatment using a plasma is shown schematically in FIG. 7. In atreatment chamber 510, a (pressed) substrate 11 to be treated rests on asupport 501. The chamber 510 is provided with a vacuum pump 511 toevacuate the atmosphere from the chamber 510, and with a gas inletconnection 512 to which a gas cylinder can be connected to set anatmosphere of a desired pressure and composition in the chamber 510. Thechamber 510 is provided with means 520 for generating electromagneticradiation of a frequency of 2450 MHz and a power of 600 W (comparable tomicrowave oven radiation), so that the gas in the chamber 510 ionizes,at least in the proximity of the substrate 11. Since such means 520 areknown per se, they will not be discussed further. In a practical test,in the chamber 510 a gas atmosphere was set, of a pressure ofapproximately 1 mbar, and a composition of substantially 95% O₂ and 5%CF₄. The substrate 11 had been manufactured and pressed in the mannerdescribed hereinbefore; the polymer contained 50% graphite powder. Thesubstrate 11 was exposed to the formed plasma for 2 minutes. Thereafter,in a manner comparable to that discussed earlier, in an electroplatingbath a lead film 31 of a thickness of about 25 μm was applied. This leadfilm 31 was found to be completely (100%) closed, to have a goodhomogeneity, and to endure the above-mentioned peeling test unaffected.

Both surface activating methods have their own merits.

An important advantage of the corona method is that the necessaryequipment is relatively simple and relatively inexpensive. Further, thecorona has a corrosive action, so that a microroughness is caused.

The plasma method, however, has many advantages over the corona method.One advantage to be mentioned is that the plasma method is intrinsicallysafe in that no high electrical voltages are needed. Process-technicaladvantages of the plasma method include:

the chamber 510 is closed off during the treatment, so that atmosphericcontamination is precluded and the reproducibility of the process ishigh;

the process time is fairly short;

the energy consumption is relatively low;

the process enables a batchwise treatment which links up well with thebatchwise pressing process of the substrates;

the treatment is not limited to flat surfaces.

Although a corona process, as such, has the advantage of simplicity, itbeing possible to carry out the process in “ordinary” ambient air, theend result achieved (lead-plated substrate) has a better quality in aplasma process, viz. a 100% coverage of the lead film. It has been foundthat the lead film, after activation by a corona process, has amicroporosity which is imputed to dust particles in the air. Althoughthis microporosity is very slight (less than 1%), it can still mean areduction of the life of a battery. It is expected that suchmicroporosity can be avoided by filtering the air to be used in thecorona process, but the required filtering equipment then detracts fromthe above-mentioned simplicity of the corona process.

By contrast, the use of a closed chamber in the plasma process providesthe advantage that the composition of the gas atmosphere used can beaccurately set. In the example discussed, CF₄ has been added to the gas.It has been found that, as a consequence, the activation of a plasmaprocess is effective for many months, probably even more than a yearafter the treatment. The activation of a corona process is found to beeffective up to about 50 hours after the treatment. This can be animportant datum if it is desired not to perform the plating treatmentimmediately after the activation treatment, but to allow some time toelapse between the two operations, for instance because the activationon the one hand and the plating on the other take place atgeographically separate locations.

It is not possible to use CF₄ in a corona process because the coronaprocess occurs under atmospheric conditions, and the CF₄ is particularlyreactive.

It is noted, in passing, that the use of CF₄ in the plasma process isnot requisite. When using a 100% O₂ atmosphere, however, the activationachieved will be effective only for a limited time (about 50 hours).

In view of the above-mentioned advantages, activation through a plasmais preferred, in particular because then the lead films 31, 32 areclosed with certainty, which accordingly affords a better protection tothe graphite present in the substrate 11. As a consequence, it is alsopossible to select the concentration of the graphite in the substrate 11to be higher. Defined as a parameter here is the weight of the graphitepowder as a percentage of the weight of the polymer powder in thesolvent (during an experiment, 100 grams of polymer powder weredissolved in 500 ml acetone). Tests were done at graphite concentrationsof 40%, 50% and 60%, which invariably yielded good results; higherpercentages seem to be possible too.

The advantage of such high percentages is twofold. In the first place,the graphite powder contributes to the conductivity of the substrate: ina test with substrates whose graphite concentrations were 40% and 50%, asurface specific resistance of about 0.04 Ωcm² was measured, while in atest with a substrate whose graphite concentration was 60%, a surfacespecific resistance of about 0.02 Ωcm² was measured. In the secondplace, graphite particles which, after the thermal pressing, are locatedadjacent the surface, can function as nucleation sites for the leaddeposition process. The more of such nucleation sites the substrate has,the easier it is to form a completely closed lead film.

In a preferred embodiment, a substrate 30 lead-plated according to thepresent invention has two lead films 31, 32 of mutually differentthicknesses. For instance, the thickness of one lead film 31 is about 40μm, while the thickness of the other lead film 32 is approximately 20μm. The thicker film 31 will be positive in the later battery, and willbe anodically attacked by the electrolyte, while the thinner film 32will be negative in the later battery, and will be cathodicallyprotected by the electrolyte.

In this connection, it is noted that the thickness of the lead films 31,32 is a compromise between life (a greater thickness gives a longerlife) and specific performance (a greater thickness means a greaternon-productive weight).

EXAMPLE

By way of test example, a 4V bipolar battery 1 was constructed, havingthe configuration illustrated in FIG. 1B. In a container 2, successivelya positive monopolar plate 4, a bipolar plate 10, and a negativemonopolar plate 5 were arranged in series, and those plates 4, 10, and 5were separated from each other by two glass fiber separators 21 and 22.For the negative monopolar plate 5 and the positive monopolar plate 4,use was made of two pre-formed conventional starter plates of athickness of 1.7 mm. The bipolar plate 10 had the structure discussedwith reference to FIG. 1A. The bipolar plate 10 comprised a lead-platedcomposite substrate 30, the lead films 31 and 32 having a thickness ofapproximately 25 μm. For the negative plate 112, use was made of apre-formed starter plate which was identical to the negative monopolarplate 5. For the positive plate 113, use was made of a pre-formedstarter plate which was identical to the positive monopolar plate 4. Thetwo glass fiber separators 21 and 22, prior to compression, each had athickness of 2.34 mm, and they were compressed by approximately 32% uponplacement in the container 2 for assembling the configurationillustrated in FIG. 1B.

The effective surface of the plates 4, 10 and 5 was approximately 40cm²; the internal resistance of the (fully charged) battery 1 wasapproximately 0.02 Ohm.

The capacity of the thus constructed test battery 1 was measured as afunction of the discharge current density. The results are shown in thetable of Appendix I. At a specific energy of 5 Wh/kg, the specific powerwas found to be 500 W/kg. The last-mentioned figures correspond to adischarge time of 36 sec, that is, a C-rate of 100 W/Wh (ratio ofspecific power/specific energy).

It has thus been demonstrated that the present invention provides asubstrate having superior properties, which enables manufacture of abipolar plate of superior performance. In the tests discussed, thesubstrate provided by the invention was combined with standard availablestarter plates 4, 5, 112, 113; with these, already a specific power of500 W/kg was achieved in a simple manner. These standard availablestarter plates, however, are optimized for their current application,viz. monopolar use, where the plates are self-supporting and the currentis drawn along an upper edge, for which reason the grids of the platesmust be of rather thick and heavy design. By the use of plates in whichthe grid is less heavy, the performance per unit of weight will increasegreatly.

It will be clear to those skilled in the art that the scope ofprotection of the present invention as defined by the claims is notlimited to the embodiments represented in the drawings and discussed,but that it is possible to alter or modify the represented embodimentsof the method according to the invention within the scope of the conceptof the invention. Thus, for instance, the specified dimensions of thecomponents of the battery are not critical.

Further, a bipolar plate is not exclusively suitable for use in abipolar battery. The bipolar plate can also be used, for instance, in abipolar condenser or a fuel cell, where it provides the same advantagesas discussed in the foregoing.

APPENDIX I Idisch i pulses t (disch. Qdisch |A| |A/cm²| 70 ms |s| |Ah|<Vdisch> W Wh Wh/kg W/kg  4 0.1 11650 815.5 0.894 3.50 14 3.13 21.5 96 8 0.2 3960 277.2 0.616 3.50 28 2.16 14.8 192 16 0.4 1050 73.5 0.3273.50 56 1.14 7.8 384 24 0.6 500 35.0 0.233 3.20 77 0.75 4.8 527 32 0.8276 19.3 0.172 3.10 99 0.53 3.7 680 40 1.0 190 13.3 0.148 2.85 114 0.422.9 782 48 1.2 117 8.2 0.109 2.80 134 0.31 2.1 922 56 1.4 84 5.9 0.0912.50 140 0.23 1.6 960 64 1.6 60 4.2 0.070 2.30 147 0.16 1.1 1008

What is claimed is:
 1. A method for manufacturing a metallized compositesubstrate (30) suitable for use in a bipolar plate (10) comprising thesteps of: providing a composite substrate (11) in the form of a plate,with main surfaces (17,18) in the form of electrically conductingcarrier material (15); applying a thermoplastic plastic material (16) inthe pore spaces of the matrix; subjecting the combination of the carriermaterial (15) and the thermoplastic material (16) to a thermal pressingprocess; subjecting the main surfaces (17,18) of the combination of thecarrier material (15) and the thermoplastic material (16) to surfaceactivating treatment, and thereafter applying a metal film (31,32) tothe main surfaces (17,18) of the substrate; wherein the carrier materialis formed by a matrix of a carbon fiber structure which is electricallyconductive in all directions and which carbon fibers are graphitized;and performing the surface activating treatment in a dry condition.
 2. Amethod according to claim 1, wherein the surface activating treatment isperformed utilizing a plasma, generated in an atmosphere ofsubstantially oxygen, at a pressure of about 1-2 mbar.
 3. A methodaccording to claim 2, wherein CF₄ is added to the atmosphere.
 4. Amethod according to claim 1, wherein graphite powder is added to theplastic material in an amount, based on weight, which is more than 20%of the amount of plastic material (16).
 5. A method according to claim1, wherein it is ensured that the main surfaces (17,18) of themetallized composite substrate (30) have a degree of roughness of atleast about 10-20 μm.
 6. A metallized composite substrate (30), suitablefor use in a bipolar plate (10), comprising a matrix (15) of carriermaterial, electrically conducting in all directions, said carriercomprising graphitized carbon fibers (14), with a mixture ofthermoplastic plastic material (16) and graphite powder applied in thepore spaces of the matrix (15), and the amount of graphite powder in thethermoplastic plastic material (16) being at least 20% metal films(31,32) applied to the main surfaces (17,18) of the composite substrate(30), the thickness of the metal films (31,32) being 100 μm at amaximum; the metal film (31,32) being substantially homogeneous andcompletely closed; the specific surface resistance being better thanabout 0.05 Ωcm²; and wherein the mechanical adhesion of the metal films(31,32) to the main surfaces (17,18) of the composite substrate (30) issuch that they endure unaffected a peeling test with an adhesive strip(40) whose adhesive strength is 113 N per cm of strip width, in whichpeeling test a loose end portion (41) of that adhesive strip (40) issubjected to a force (F) in a direction perpendicular to the surface. 7.A metallized composite substrate according to claim 6, wherein thethickness of a first metal film (31) is approximately 40 μm and thethickness of a second metal film (32) is approximately 20 μm.
 8. Amethod for manufacturing a bipolar plate (10), comprising the steps of:manufacturing a substrate (11;30) by providing a composite substrate(11) in the form of a plate, with main surfaces (17,18) in the form ofelectrically conducting carrier material (15); applying a thermoplasticplastic material (16) in the pore spaces of the matrix; subjecting thecombination of the carrier material (15) and the thermoplastic material(16) to a thermal pressing process; subjecting the main surfaces (17,18)of the combination of the carrier material (15) and the thermoplasticmaterial (16) to surface activating treatment, and thereafter applying ametal film (31,32) to the main surfaces (17,18) of the substrate;wherein the carrier material is formed by a matrix of a carbon fibrestructure which is electrically conductive in all directions and whichcarbon fibres are graphitized; and performing the surface activatingtreatment in a dry condition; manufacturing a negative pasted plate(112) by providing a suitable paste (121) in a metal grid (120);manufacturing a positive pasted plate (113) by providing a suitablepaste (123) in a metal grid (122); and fitting the two pasted plates(112, 113) on opposite sides against the substrate (11), such thatelectrically conducting contacts are effected between the facing mainsurface of the substrate and the respective pasted plates.
 9. A methodaccording to claim 8, wherein it is ensured that the pasted plates (112,113) are pressed against the main surfaces (17, 18) by locking theplates (112, 113) between two compressed glass fiber mats (21).
 10. Amethod according to claim 8, wherein a chemical connection between theplates (112, 113) and the respective surfaces (17, 18) of the substrate(11; 30) is effected.
 11. A method according to claim 8, wherein saidchemical connection is effected by fitting the plates (112, 113) againstthe substrate (11; 30) directly after the paste (121, 123) has beenprovided in the respective grid (120, 122), and allowing the curingprocess to take place after the plates (112, 113) have been fittedagainst the substrate (11; 30).
 12. A method according to claim 8,wherein the metal grid (120, 122) is a metal foam.
 13. A bipolar plate(10) comprising: a metallized composite substrate (30) comprising amatrix (15) of carrier material, electrically conducting in alldirections, said carrier comprising graphitized carbon fibers (14), witha mixture of thermoplastic plastic material (16) and graphite powderapplied in the pore spaces of the matrix (15), and the amount ofgraphite powder in the thermoplastic plastic material (16) being atleast 20%; metal films (31,32) applied to the main surfaces (17,18) ofthe composite substrate (30), the thickness of the metal films (31,32)being 100 μm at a maximum; the metal film (31,32) being substantiallyhomogeneous and completely closed; the specific surface resistance beingbetter than about 0.05 Ωcm²; and wherein the mechanical adhesion of themetal films (31,32) to the main surfaces (17,18) of the compositesubstrate (30) is such that they endure unaffected a peeling test withan adhesive strip (40) whose adhesive strength is 113 N per cm of stripwidth, in which peeling test a loose end portion (41) of that adhesivestrip (40) is subjected to a force (F) in a direction perpendicular tothe surface; a negative pasted plate (112) arranged against the firstmain surface (17) of the composite substrate (30), which comprises ametal grid (120) and a paste (121) provided therein; a positive pastedplate (113) arranged against the second main surface (18) of thecomposite substrate (30), which comprises a metal grid (122) and a paste(123) provided therein.
 14. A method for manufacturing a bipolarbattery, wherein there are arranged in a container (2) a series of (a) amonopolar positive pasted plate (4), (b) at least one bipolar system(10), which comprises in succession a negative plate (112), a metallizedcomposite substrate (30), and a positive plate (113), and (c) amonopolar negative pasted plate (5), and wherein between the monopolarpositive pasted plate (4) and the adjacent bipolar system (10), betweenthe monopolar negative pasted plate (5) and the adjacent bipolar system(10), and between the bipolar systems (10) mutually, if more than one ispresent, a glass fiber separator (21) is positioned, which is filledwith a suitable electrolyte, the thickness of the glass fiber separators(21) being chosen such that, after being positioned, they are compressedby about 30-35%.
 15. A method according to claim 14, wherein themonopolar negative pasted plate (5) is identical to the negative plate(112) of the bipolar system (10), and wherein the monopolar positivepasted plate (4) is identical to the positive plate (113) of the bipolarsystem.
 16. A method according to claim 14, wherein a stack of,successively, a first pasted plate (112), a separator (21), and a secondpasted plate (113), is provided in a recess (214, 212) in a frame likeplastic spacer (200).
 17. A method according to claim 16, wherein thethickness of said stack is greater than the thickness of the spacer. 18.A method according to claim 16, wherein between successive spacers (200)at least one substrate (11; 30) is provided.
 19. A method according toclaim 18, wherein the dimensions of the substrates (11; 30) aresubstantially equal to those of the spacers (200).
 20. A methodaccording to claim 19, wherein the series of spacers (200) andsubstrates (11; 30) is compressed to contact the spacers (200) and thesubstrates (11; 30) with each other, whereby the separators (21) undergoa compression of preferably about 30 to 35%, and whereby thecircumferential edges of the spacers (200) and the substrates (11) arejoined together in liquid-tight manner.
 21. A method according to claim18, wherein between successive spacers (200) two substrates (11; 30) arearranged, and wherein between those two successive substrates (11; 30) acooling element (300) is arranged.
 22. A bipolar battery (1; 100),manufactured by a method wherein there are arranged in a container (2) aseries of (a) a monopolar positive pasted plate (4), (b) at least onebipolar system (10), which comprises in succession a negative plate(112), a metallized composite substrate (30), and a positive plate(113), and (c) a monopolar negative pasted plate (5), and whereinbetween the monopolar positive pasted plate (4) and the adjacent bipolarsystem (10), between the monopolar negative pasted plate (5) and theadjacent bipolar system (10), and between the bipolar systems (10)mutually, if more than one is present, a glass fiber separator (21) ispositioned, which is filled with a suitable electrolyte, the thicknessof the glass fiber separators (21) being chosen such that, after beingpositioned, they are compressed by about 30-35%, wherein at least aspecific energy of 5 W/kg, the specific power is at least about 500W/kg.
 23. The method of claim 1, wherein the composite substrate issuitable for use in a bipolar battery.
 24. The method of claim 1,wherein the metal film is a lead film.
 25. The method of claim 1,wherein the surface activating treatment is performed by utilizing acorona discharge or a plasma.
 26. The method according to claim 3,wherein CF₄ is added in an amount of about 5%.
 27. The method accordingto claim 4, wherein the graphite powder comprises at least 40% of theamount of plastic material.
 28. The method according to claim 27,wherein the graphite powder comprises at least 60% of the amount ofplastic material.
 29. The substrate of claim 6, wherein the graphitepowder comprises at least 40% of the amount of plastic material.
 30. Thesubstrate of claim 29, wherein the graphite powder comprises at least50% of the amount of plastic material.
 31. The substrate of claim 6,wherein the metal films are lead films.
 32. The substrate of claim 6,wherein the thickness of the metal films is 40 μm at maximum.
 33. Thesubstrate of claim 6, wherein the specific surface resistance is greaterthan about 1.025 Ωcm².
 34. The method according to claim 12, wherein themetal foam is a lead foam or a lead alloy foam.
 35. The method accordingto claim 16, wherein the separator is manufactured from glass fiber.